Solution for solving PVC pipe quality problem

Solution for solving PVC pipe quality problem

In the production of PVC pipe production, various quality problems may be caused by factors such as raw material formula, equipment operation, process parameters or improper operation. 

The following is a detailed analysis of common faults, possible causes and solutions:

1. Appearance defects of pipe materials

1. Rough surface, no luster

Possible reasons

· The raw material is not well plasticized (the temperature is too low or the screw speed is too fast).

· The mold runner is not designed reasonably, and the melt flow is not smooth.

· Insufficient or uneven distribution of lubricant leads to excessive friction between melt and equipment.

· The cooling water temperature is too high, the surface of the pipe material solidifies quickly but the interior is not fully plasticized.

 resolvent

· Increase the temperature of each section of the extruder (especially the feeding section and compression section), or reduce the screw speed to extend the plasticizing time.

· Check whether the mold runner has dead corners or insufficient roughness, and polish or modify the mold structure if necessary.

· Adjust the amount of lubricant (such as stearic acid, paraffin) to ensure that it is fully mixed with the resin.

· Reduce the water temperature of the cooling tank (it is recommended to control it at 15~25℃), or increase the cooling length.

2. Bubbles or pores appear on the surface

Possible reasons

· The moisture content of raw materials is too high (PVC the resin or additive is absorbed).

· The exhaust hole of the extruder is blocked, and the volatilization (such as water and monomer) cannot be effectively discharged.

· The processing temperature is too high, resulting in PVC decomposition and gas generation (with pipe yellowing).

 resolvent

· The raw material is dried (e.g., PVC, resin is dried at 80℃ for 2 hours) to ensure that the moisture content is less than 0.1%.

· Clean the exhaust hole, check whether the vacuum pump works normally, and keep the exhaust pressure stable.

· Reduce the processing temperature (especially the die head and mold temperature), check whether the temperature control instrument is faulty.

3. The surface has scratches or dents

Possible reasons

· There are debris, burrs and scratches on the surface of the pipe in the fixed diameter sleeve or cooling water tank.

· The traction speed does not match the extrusion speed, resulting in excessive stretching or accumulation of pipe.

· The gap between the mold die and the core die is not uniform, and the force of melt extrusion is not uniform.

 resolvent

· Stop the machine to clean the sizing sleeve and cooling water tank, check the smoothness of the inner wall, and replace the worn parts when necessary.

· Adjust the traction speed (usually 1.0~1.2 times of the extrusion speed) and synchronize control through the frequency converter.

· Re-calibrate the die clearance, use a feeler gauge to measure the concentricity of the die and core to ensure uniform discharge.

4. Cracks appear on the inner wall of the pipe (inner crack)

Possible reasons

· The core temperature is too high, resulting in the early curing of the inner wall of the pipe, and cracking under tensile stress during cooling.

· Internal cooling is insufficient (such as core mold cooling waterway blockage), the inner wall heat dissipation is slow, low strength.

· The traction speed is too fast, and the longitudinal tensile stress is generated inside the pipe.

 resolvent

· Reduce the core mold temperature (5~10℃ lower than the die mold temperature), or increase the cooling water of the core mold.

· Clean the cooling channel of the core mold to ensure smooth water circulation.

· Reduce the drawing speed to make the pipe fully shaped in the die (drawing speed ≤ extrusion line speed).

 

2. The size of the pipe is not up to standard

1. The pipe diameter deviation is too large

Possible reasons

· The size of the mold mouth is worn (the inner diameter becomes larger after long-term use).

· Traction speed fluctuation (such as traction machine belt slip, motor speed instability).

· The cooling effect is insufficient, and the pipe diameter changes due to the sagging of the pipe before cooling.

 resolvent

· Check the wear of the mold regularly, and replace or repair the die (tolerance control is ±0.05mm).

· Overhaul the transmission system of the traction machine, replace the belt or adjust the closed-loop control of the motor speed.

· Increase the length of the cooling water tank or increase the cooling water quantity to ensure that the pipe is fully shaped before entering the traction machine.

2. Uneven wall thickness

Possible reasons

· The core die of the mold is not concentric with the mouth die, resulting in more material on one side and less material on the other.

· The screw speed is too fast, the melt rotates unevenly in the barrel, resulting in circumferential pressure difference.

· The filler (such as calcium carbonate) in the raw material formula is not evenly dispersed and has poor local fluidity.

 resolvent

· Re-adjust the concentricity of the mold and calibrate the position of the core mold through the dial gauge.

· Reduce the screw speed (usually controlled at 15~30r/min) to improve the melt homogenization effect.

· Optimize the mixing process (such as extending the high speed mixing time to 10~15 minutes) to ensure uniform dispersion of filler.

3. Insufficient mechanical properties

1. The tensile strength or impact toughness of the pipe material is not up to standard

Possible reasons

· PVC The resin model is not selected properly (for example, if SG-5 type resin is used to produce thick wall pipe, high molecular weight SG-3 type should be selected).

· The amount of stabilizer and toughening agent (such as CPE and ACR) is insufficient or the quality is poor.

· Excessive or insufficient plasticization: the temperature is too high to cause resin degradation, and the temperature is too low to fully entangle the molecular chain.

 resolvent

· Select the appropriate model of resin according to the use of pipe (such as SG-6 for water supply pipe, SG-5 for drainage pipe).

· Increase the amount of stabilizer (such as calcium-zinc composite stabilizer) and toughening agent, and replace raw materials from high-quality suppliers.

· Optimize the temperature curve and monitor the plasticization effect through the melt pressure sensor (the ideal melt pressure is 8~15MPa).

2. The brittleness temperature of pipe material is high (cracking at low temperature)

Possible reasons

· Insufficient amount of plasticizer (e.g. DOP) added, or volatile plasticizer used.

· During the processing, the shear overheating leads to the fracture of resin molecular chain and the decrease of molecular weight.

· High filler content (e.g., calcium carbonate over 30%) reduces material flexibility.

 resolvent

· The amount of plasticizer should be appropriately increased (taking into account the hardness requirements), or low volatile plasticizer (such as DOS) should be used instead.

· Reduce the gap between screw and barrel (when the gap is more than 0.5mm after wear, it needs to be replaced) to reduce shear heating.

· Control the filler content, or use nanoscale fillers (such as nano-calcium carbonate) to improve compatibility.